1. Field of the Invention
This invention relates to a stepper motor controller, particularly, but not exclusively, to a stepper motor controller for use in a lighting unit for producing decorative effects.
2. Description of the Related Art
Stepper motors are commonly used in applications where the angular position of an object is to be indexed with a high degree of accuracy. An exemplary stepper motor is illustrated schematically in FIG. 1 appended hereto. The motor 10 has a magnetized rotor 12 housed in a stator 14 comprising an array of coils 16. The rotor 12 is mounted to rotate in the stator 14.
The coils 16 of the stator 14 are organised into sets in the stators schematically illustrated in FIG. 1 as two centrally tapped coils. The central taps of the coils are connected in common to ground, thereby rendering the coils effectively four independent sets 16A, 16B, 16C, 16D, which can be energized progressively to urge the rotor into rotating. In particular, the coils of the rotor 12 are energised by means of four input lines 18A, 18B, 18C, 18D. The input lines 18A, 18B, 18C, 18D are connected to a stepper controller (not shown) which applies suitable signals to the input lines 18A, 18B, 18C, 18D so as to set up rotation of the rotor 12.
In a conventional stepper controller, the energizing input signals are square waves, the square waves being shifted in phase relative to each other to establish a progressive energising of the coils 16 of the stator. A typical set of waveforms, for application to each of the input lines 18A, 18B, 18C, 18D, is illustrated in FIG. 2 appended hereto. In FIG. 2, the period labelled .lambda. is one cycle of the driver signals to the motor.
The waveforms as illustrated are relatively straightforward to produce, either by digital electronics or by a software controlled driver. However, they are disadvantageous in that they comprise a series of sharp edges which constitute discontinuities. That leads to rather discontinuous and jerky motion of the rotor. Moreover, the resolution of the motor is limited, in that the angular position of the rotor can only be selected to the nearest half step of the motor.
In order to overcome the problem, a controller has been built which applies signals of selected shape to the input lines 18A, 18B, 18C, 18D. The signals are constructed by pulse width modulation (PWM).
The four input lines 18A, 18B, 18C, 18D each have applied thereto an individually constructed waveform. The waveform consists of a series of pulses, each pulse being a positive half of a sinusoid. The controller as described requires four PWM drivers, or two bipolar PWM drivers, in order to construct the array of input signals. An exemplary set of signals is illustrated in FIG. 3 of the appended drawings. In FIG. 3, the period labelled .lambda. is one cycle of the driver signals to the motor.